The reduction of prolactin release caused by the neurohormone dopamine (DA) is the most thoroughly understood system of any of the hypothalamic releasing or inhibiting factors. We have measured DA receptors in the normal anterior pituitary (AP), but have failed to observe DA receptors in two AP derived prolactin secreting clones (GH3 & 2351). In contract, we recently found normal DA receptors in transplantable AP turmors 7315a and MtTW15 which secrete prolactin but are refractory to DA inhibition of prolactin release. We shall continue to characterize these DA receptors and prove that they are coincident with the prolactin cells of the tumors. Furthermore, cell lines and clones will be developed from these tumors and these cells will hopefully continue to express both the DA receptor and the prolactin gene. This would allow fine biochemical studies of the proximal events associated with DA receptor activation and the site of the functional lesion may be exposed. We know that both tumors have coupling or G/F protein because of the ability to decrease agonist affinity with certain guanine nucleotides. This G/F protein probably couples the DA receptor to second messenger. We have also determined that both clones and tumors contain an adenylate cyclase. There is now good evidence that DA receptor activation inhibits adenylate cyclase in the normal AP. Therefore, we shall characterize this property in the AP and then test to see if this response still exists in the tumors and clones. Several recent findings indicate that the DA receptor assay, as it is now applied to the AP, may be inadequately controlled. We shall utilize our experience with this assay and determine the extent of these possible artifacts (e.g. ascorbic acid, EDTA). In addition, this assay will be used to test whether calcium and liposome antagonist drugs, which inhibit DA receptor binding, interact with the DA receptor in a competitive or noncompetitive manner. In conclusion, the derangement of the DA inhibitory control mechanism causes the most prevalent AP tumor in humans, the prolactin secreting adenoma. Thus, the elucidation of mechanisms linking the DA receptor to the prolactin secretory system has tremendous basic and clinical relevance.